Elemental analysis (EA) involving combustion/reduction and/or pyrolysis is a widely used technique for the quantification of elemental concentrations of hydrogen, carbon, nitrogen, oxygen, and sulphur in organic and inorganic materials. Interfaced with isotope ratio mass spectrometry (IRMS), EA is commonly employed as a technique for the measurement of stable isotope ratios of the aforementioned elements. See Calderone et al, (2004) J. Agric. Food Chem., 52, 5902-5906, and Fadeeva et al, (2008) J. Anal. Chem. 63(11), 1094-1106, the disclosures of both references being hereby incorporated by reference in their entirety (however, where anything in the incorporated references contradicts anything stated in the present application, the present application prevails). The general way of reporting stable isotope ratios from EA-IRMS analysis is using “delta notation” (δ-notation). IRMS allows precise measurement of isotope abundance in a sample gas, which allows determination of a ratio of the heavy to light isotope (R), such as:
  R  =                    Heavy        ⁢                                  ⁢        Isotope                    Light        ⁢                                  ⁢        Isotope              =                                                     13                    ⁢          C                                                   12                    ⁢          C                    =                                                                 15                        ⁢            N                                                             14                        ⁢            N                          =                                                           18                        ⁢            O                                                             16                        ⁢            O                              The δ-notation is the stable isotope ratio of an unknown sample relative to a standard (i.e. reference material) of known isotope value, calculated as:
      δ    ⁡          [      %      ]        =                                          R                          (              Sample              )                                -                      R                          (              Standard              )                                                R                      (            Standard            )                              *      1000        =                  (                                            R                              (                Sample                )                                                    R                              (                Standard                )                                              -          1                )            *      1000      
For EA-IRMS, two approaches are typically used. One approach uses two reactors (hereafter Approach 1), where the combustion and reduction processes occur in separate reactors heated by different furnaces or alternatively by the same furnace. Another approach combines the combustion and reduction processes into the same reactor (hereafter Approach 2), which is heated by the same furnace.
The analysis of hydrogen, carbon, nitrogen and sulphur is achieved by combusting a sample matrix in a reactor held at a temperature in a range of between 400° C. and 1,100° C., but ideally in a range of between 950° C. and 1,100° C. Generally, samples are sealed in tin capsules (alternatively, silver or aluminum capsules) and introduced to the combustion reactor by an autosampler in a flow of carrier gas, such as helium (for EA-IRMS), or argon or nitrogen (EA analysis only). The carrier gas flow maintains pressure and temperature regimes in the reactor, minimizes introduction of contaminant gases, such as air, and reduces damage to any materials or chemicals inside the system. Elemental analyzers require carrier flows of up to 1,000 ml/min depending on the volume to be flushed, the largest volume generally being the reactor. The typical operating flow rate through the reactor, however, is in the range of between 0.2 ml/min and 300 ml/min, such as in the range of between 40 ml/min and 300 ml/min, or between 40 ml/min and 200 ml/min (EA analysis only), depending on the reactor size, and a purge flow to the autosampler is generally in a range of between 20 ml/min and 300 ml/min. At or near the point of sample introduction, an injection of oxygen gas (O2) may or may not occur, depending on the sample matrix, to support the combustion process (Lott et al, Rapid Commun. Mass Spectrom. 2015, 29, 1-8), hereby incorporated by reference in its entirety (however, where anything in the incorporated reference contradicts anything stated in the present application, the present application prevails). At this point, the sample matrix breaks down, and is conveyed by the carrier gas across an oxygen donor compound (e.g., Cr2O3, WO3), which is designed to ensure complete oxidation of the carbon, nitrogen and sulphur elements evolved from the sample matrix to gaseous oxidized products (e.g., CO2, NOx, SO2). After the oxidation process, the next step in the reaction varies depending on the set-up of the system. In Approach 1 (two reactor system), the gases are then optionally swept across a sulphur/halogen trap and transferred to a reduction reactor, typically via a stainless steel/sulfinert capillary or heated bridge, which contains metallic copper. The reduction reactor is generally held at a temperature in a range of between 450° C. and 900° C., and is designed to reduce NOx (x=1, 2, 3) gas species to N2, reduce SO3 to SO2 and absorb excess O2 not used in the combustion reaction. After the gases are carried out of the reduction reactor, they are swept through a water trap (e.g., magnesium perchlorate) and/or optionally a CO2/acid gas trap (e.g., carbosorb) before being analyzed. After leaving the reduction reactor, the gases are typically separated on a gas chromatography column or by an adsorption/thermodesorption technique prior to analysis by a thermal conductivity detector (TCD), a flame ionization detector (FID), or an isotope ratio mass spectrometer. In Approach 2, however, after the analyte gas passes across the oxygen donor compound, the oxidized gaseous products are conveyed onto metallic Cu within the same reactor, where the same chemical reaction as in Approach 1 occurs: a sulphur/halogen trap may also be present in the lower section of the reactor (e.g., AgCoO4 or silver wool). Thereafter, the gas is conveyed from the reactor through a water trap and/or optionally a CO2/acid gas trap before gas separation and detection as in Approach 1.
A stoichiometric combustion and reduction reaction pathway is necessary for the accurate and precise determination of percent elemental and isotopic measurements of carbon, nitrogen and sulphur. For nitrogen analysis, specifically, the process is designed to produce N2 gas for detection by the TCD and/or isotope ratio mass spectrometer. During the sample combustion process, nitrogen compounds are broken down and subsequently form nitrogen oxides (NOx). These oxides of nitrogen must be stoichiometrically reduced to N2, which is one of the functions of the metallic Cu in the reactor(s) described in Approach 1 and Approach 2.
There is, nevertheless, a need for further improvements in the stoichiometric reduction of NOx, species to N2 before the analyte gas is conveyed from the reactor for separation and subsequent analysis by TCD and/or IRMS.